The present invention relates generally to active matrix displays.
FIG. 1 shows a section of an active matrix display with pixel elements including light emitting diodes. The section of an active matrix display in FIG. 1 includes a matrix of pixel elements (e.g., 100AA, 100AB, 100AC, 100BA, 100BB, 100BC, 100CA, 100CB, and 100CC), an array of column conducting lines (e.g., 200A, 200B, and 200C), an array of row conducting lines (e.g., 300A, 300B, and 300C) crossing the array of column conducting lines.
A pixel element (e.g., 100BB) in the matrix of pixel elements is electrically connected to a column conducting line (e.g., 200B) and a row conducting line (e.g., 300B). The pixel element (e.g., 100BB) includes a light emitting diode 50, a driving transistor 40, a capacitive element 30, and a switching transistor 20. The light emitting diode 50 is electrically connected to a semiconductor channel of the driving transistor 40. The capacitive element 30 has a terminal electrically connected to a gate of the driving transistor 40. The gate of the driving transistor 40 is electrically connected to a column conducting line (e.g., 200B) through a semiconductor channel of the switching transistor 20. The gate of the switching transistor 20 is electrically connected to a row conducting line (e.g., 300B).
During operation, a pixel element (e.g., 100BB) generally can be either in a charging mode or in a light-emitting mode. When the pixel element (e.g., 100BB) is in the charging mode, a selection signal (e.g., a selection voltage) on the row conducting line (e.g., 300B) drives the switching transistor 20 into a conducting state. When the switching transistor 20 is in the conducting state, a data signal (e.g., a data voltage) on a column conducting line (e.g., 200B) can set a gate voltage at the gate of the driving transistor 40 to a target voltage value. When the pixel element (e.g., 100BB) is in the light-emitting mode, a deselect signal (e.g., a deselect voltage) on the row conducting line (e.g., 300B) drives the switching transistor 20 into a non-conducting state. When the switching transistor 20 is in the non-conducting state, a gate voltage at the gate of the driving transistor 40 can be substantially maintained.
In general, a driving current passing through the light emitting diode 50 is determined by the gate voltage at the gate of the driving transistor 40. But, the driving current passing through the light emitting diode 50 also depends on some individual properties of the driving transistor 40. For example, the driving current passing through the light emitting diode 50 can depend on the threshold voltage and the carrier mobility of the driving transistor 40. The driving transistor 40 in different pixel elements may have different properties. Therefore, in certain applications, it is desirable to provide a pixel element that can compensate property variations among different pixel elements.